The invention relates to a measuring apparatus for monitoring residual oxygen in an exhaust gas, to a method for operating the measuring apparatus, and also to a use of the method.
The measuring apparatus is a modified lambda probe (broad band lambda probe) such as is used in cars for monitoring the exhaust gases. The measuring apparatus includes a sensor and in the interior of this a “reaction chamber” in which the partial pressure of molecular oxygen is influenced via electrode reactions. This oxygen partial pressure can be changed or adjusted by means of an oxygen ion pump working electrochemically between the reaction chamber and an exhaust space. At the same time as a transport of oxygen ions in the ion pump, a transport of molecular oxygen takes place through a “diffusion gap” into the reaction chamber or out of it. The diffusion gap joins the reaction chamber with the measuring point in the exterior region of the probe at which the concentration of the residual oxygen in the exhaust gas (or its partial pressure) is determined as a desired value. The oxygen partial pressure in the reaction chamber is preset as the desired value of a regulating circuit. The actual value of this regulating circuit is determined electrochemically by means of a Nernst cell with respect to a reference value which is given by the oxygen partial pressure of the atmosphere. An electrical “pump current” with which an oxygen outflow or inflow is brought about in the oxygen ion pump is the set value of the regulating current. A steady state sets in in which the diffusion current of the molecular oxygen through the diffusion gap and the oxygen ion current in the ion pump are the same size. The difference between the oxygen partial pressure in the exhaust gas and that of the reaction chamber results in a pump current which can not only be positive but also negative.
The Nernst cell and the oxygen ion pump respectively include an oxygen ion conducting solid electrolyte layer and electrode layers applied on this layer by means of which redox processes result with molecular oxygen, with oxygen ions of the solid electrolyte layer and with electrons. The solid electrolytes are only conductive for the oxygen ions at a high temperature.
The electrons in the oxygen ion pump are introduced or removed via an electrical circuit at the electrodes; in this arrangement they form the pump current. In the steady state condition the strength of the pump current can be used as a measure for the concentration of the residual oxygen in the exhaust gas which is to be measured. The voltage determined using the Nernst cell is compared in an electronic measuring apparatus with a reference voltage which corresponds to the desired value of the partial pressure in the reaction chamber. The strength of the pump current is regulated to adapt the actual value to the desired value.
When using a fuel, in particular a gaseous or gasified fuel for room heating purposes, an energy saving procedure consists of producing electrical energy by means of fuel cells in addition to the production of thermal energy. In this or other electrochemical reactions such as in combustion of the fuel, precautionary measures are prescribed. A measure of this kind can be the use of a measuring apparatus for monitoring the residual oxygen in the exhaust gases which arise during the production of energy in order to avoid a less than stoichiometric combustion and a formation of explosive or toxic gases. An example of a system with a Lambda probe is described in EP-A-0 818 840 (FIG. 7). During the monitoring of the residual oxygen a reliable “intrinsically safe” functional reliability of the probe used for the measurement is of great importance. At any time during the operation of the fuel cell unit, one has to know whether the measuring apparatus is carrying out the monitoring correctly.
Diverse defects can arise when using a Lambda probe: cracks for example in the solid electrolyte layers of the ion pump and the Nernst cell; blocking of the diffusion gap with soot; an increase in the heat resistance; and a drift of the internal resistance of the Nernst cell as a consequence of aging.